Comparison between the productivity of pure and mixed stands of Norway spruce and European beech along an ecological gradient

Existing growth and yield plots of pure and mixed stands of Norway spruce (Picea abies (L.) H. Karst.) and European beech (Fagus sylvatica L.) were aggregated in order to unify the somewhat scattered sources of information currently available, as well as to develop a sound working hypothesis about mixing effects. The database contains information from 23 long-term plots, covering an ecological gradient from nutrient poor and dry to nutrient rich and moist sites throughout Central Europe. An empirically formed interaction model showed, that depending on the site conditions, dry mass growth in mixed stands can range from -46% to +138% of the growth yielded by a scaled combination of pure stands at equal mixing proportions. Drawing from the interaction model, overyielding of the mixed stands appears to be triggered by two separate mechanisms. On poor sites, where significant overyielding is commonly found, facilitation by beech offsets nutrient-related growth limitations in spruce. In contrast, overyielding of mixed stands occurs less frequently on rich sites, and appears to be based on an admixture effect, with spruce reducing the severe intra-specific competition common in pure beech stands. It was concluded that silviculture can accelerate growth of spruce by beech admixtures on poor sites, while growth of beech can be promoted by admixture of spruce, particularly on excellent sites.